Stages of Wastewater Treatment Explained

Wastewater treatment is of great importance and takes place in a number of different ways, comprising various stages.

Each stage focuses on different pollutants, and as the water moves from one phase to another, it becomes cleaner.

We shall discuss the various types and stages of wastewater treatment and what they entail in more detail below.

What is Wastewater Treatment?

Wastewater treatment is a process which is carried out in order to remove contaminants from wastewater.

Thanks to such a procedure, the wastewater will be converted to an effluent.

It can then be returned to the water cycle so that it is either reused, or else be safely released back into the natural waterways.

The treatment process comprises different stages and is carried out in a wastewater treatment plant.

In simple terms, the wastewater will be taken away and then screened.

Afterwards, the primary, secondary and tertiary treatment processes will be carried out.

Hence, whenever a toilet is flushed, or the sink or bath in your home is emptied for example, the wastewater generated will go down the drain, into a system of pipework, on to a larger sewer system situated underground.

This sewer will be connected to a whole network of other sewers, which will move the wastewater to a sewage treatment plant.

Here it will undergo numerous processes as it will be cleaned and filtered, until it is deemed to be safe to discharge in the environment or reuse.

Types of Wastewater Treatment Plants

Wastewater treatment plants vary depending on the type of wastewater that they treat.

Indeed, the processes will vary depending on the type and level of contamination of the wastewater in question.

Sewage Treatment Plants

Sewage treatment, municipal wastewater treatment or domestic wastewater treatment, all refer to a type of treatment which mainly aims at removing contaminants from the sewage.

Effluent is then produced and it will be safely discharged into the environment or reused in one or more applications.

There are different types of sewage treatment processes available, including decentralised systems like on-site treatment systems, and larger centralised systems such as the sewerage which lead the sewage to a treatment plant.

Sewage treatment generally includes two main stages, although in some cases tertiary treatment also takes place.

Over the years many sewage treatment technologies have been improved, with most using biological treatment processes

Industrial Wastewater Treatment Plants

Industrial wastewater treatment refers to those processes which are carried out to treat wastewater produced by industries.

In such cases, the wastewater is considered to be an undesirable by-product.

As a result, it needs to be properly treated in order to be converted into an effluent which can be reused or released safely.

Some industrial facilities will have their own specialised facilities to treat their respective wastewater.

These typically include chemical plants and petroleum refineries.

Some industries install a pre-treatment system in order to remove some of the pollutants.

As a result, the wastewater will be partially treated and it may be discharged to the municipal sewer system after.

Over the years there have also been several industries who managed to redesign their manufacturing processes in order to greatly reduce or even eliminate pollutants altogether.

Some treatment processes used include oil and grease removal, solids removal, brine treatment, and removal of acids, alkalis and any toxic materials.

Agricultural Wastewater Treatment Plants

Agricultural wastewater treatment plants are important as such wastewater may include chemicals found in fertilisers, pesticides, irrigation water, and crop residues, among others

The treatment process can be carried out in plants that make use of mechanised treatment units.

In the case of animal slurry, it usually gets treated by containment in anaerobic lagoons prior to disposal.

In some cases constructed wastelands are used to facilitate the treatment of animal waste.

Leachate Treatment Plants

Leachate treatment plants are used in order to treat leachate from landfills.

There are various treatment options that can be used, including mechanical treatment by ultrafiltration, biological treatment and using active carbon filters.

1. Primary Wastewater Treatment

During the primary treatment, the wastewater will be temporarily kept in a settling tank.

During this phase the heavier solids will have time to sink down to the bottom, whereas the lighter solids start to float up to the surface.

These materials will then be held back in the tank, while the remaining liquid gets discharged to the secondary treatment phase.

The tanks are equipped with mechanical scrapers in order to continually move the settled solids, or the sludge that gets collected in the base to a hopper.

This will in turn pump it to a sludge treatment facility.

2. Secondary Wastewater Treatment

The secondary wastewater treatment stage is more rigorous than the former stage.

In fact this phase comprises three processes.

During the secondary stage, there will be a substantial degradation of the biological content that’s present in the waste by means of what are scientifically referred to as aerobic biological processes.

During such processes, useful bacteria will help to break down and remove the harmful bacteria.

Once this phase is completed, the wastewater will be much safer as the common biodegradable contaminants will have diminished to safe levels.

Thus, it can be released in the local environment relatively safely.

The secondary wastewater treatment is made up of three elements, namely:

Biofiltration

By making use of sand filters and contact filters, any additional sediment is effectively removed from the wastewater.

Aeration

Aeration is a rather long process whereby the oxygen saturation is increased by introducing air to the wastewater.

This process is very effective but it can take several hours, sometimes even up to 30 hours, to be completed.

Oxidation Ponds

This method makes use of natural bodies of water.

The wastewater is allowed to pass through for a certain period, and then it is retained for two to three weeks.

This method is mostly used in areas where there are warmer climates.

3. Tertiary Wastewater Treatment

The tertiary wastewater treatment phase is mainly intended to improve the quality of the water, either to meet specific requirements for safe discharge, or for domestic or industrial standards.

During this phase the removal of pathogens is carried out if the water is required to be safe for drinking.

Conclusion

Wastewater treatment is a long process which comprises several steps.

Over the years it has become more important to ensure that wastewater is adequately treated to avoid causing any harm to the environment.

There are also various regulations in place to ensure that wastewater is treated effectively prior to being reused or safely discharged.